The present invention relates generally to retroreflective articles such as sheeting. More particularly, the invention relates to such articles or sheetings in which retroreflective elements comprise reflective faces arranged to form a cavity.
The reader is directed to the glossary at the end of the specification for guidance on the meaning of certain terms used herein.
Cube corner retroreflective sheetings can generally be categorized as those that use a rear-surface body layer and those that use a front-surface body layer. Commercially available cube corner retroreflective sheetings are of the former type, in which a thin transparent body layer has a substantially planar front surface and a rear structured surface comprising a plurality of geometric structures of pyramidal shape, some or all of which include three reflective faces configured as a cube corner element. Light is incident on the planar front surface, passes through the thickness of the body layer, and is retroreflected by the cube corner elements back through the front surface. In some cases, a reflective coating such as aluminum is applied to the rear structured surface, followed by an adhesive layer that covers and conforms to some extent to the shape of the structured surface. However, in general no reflective coating is required so long as a clean air interface can be maintained at the structured surface, in which case reflections occur by total internal reflection.
Some known cube corner retroreflective sheeting constructions use a front-surface body layer, in which the body layer has a front structured surface. See, e.g., U.S. Pat. No. 3,712,706 (Stamm), U.S. Pat. No. 4,127,693 (Lemelson), and U.S. Pat. No. 4,656,072 (Coburn, Jr. et al.), and PCT Publication WO 89/06811 (Johnson et al.). The front structured surface comprises a plurality of reflective faces arranged to form cube corner cavities. For this reason such retroreflective sheeting is referred to herein as cube corner cavity based retroreflective sheeting. A thin metal film is commonly applied to the structured surface to enhance reflectivity of the faces. Incident light does not penetrate through the body layer but rather is reflected by the faces forming the cube corner cavities. In some embodiments a cover layer that does transmit incident light is provided on top of the structured surface to protect the cavities from dirt or other degradation, with portions of the cover layer extending into and filling in the cube corner cavities of the structured surface. In other embodiments a cover layer is sealed or adhered to the structured surface by a colored pressure-or heat-sensitive adhesive that cancels, removes, or obliterates retroreflectivity of the structured surface.
Some structured surface geometries define both cube corner pyramids and cube corner cavities. An example is a structured surface that has a plurality of contiguous square faces, each being oriented mutually perpendicular to its adjacent faces, and each group of three adjacent faces having a hexagonal outline in top plan view.
Cube corner retroreflective sheeting is commonly produced by first manufacturing a master mold that has a structured surface, such structured surface corresponding either to the desired cube corner element geometry in the finished sheeting or to a negative (inverted) copy thereof, depending upon whether the finished sheeting is to have cube corner pyramids or cube corner cavities (or both). The mold is then replicated using any suitable technique such as conventional nickel electroforming, chemical vapor deposition, or physical vapor deposition to produce tooling for forming cube corner retroreflective sheeting by processes such as embossing, extruding, or cast-and-curing. U.S. Pat. No. 5,156,863 (Pricone et al.) provides an illustrative overview of a process for forming tooling used in the manufacture of cube corner retroreflective sheeting. Known methods for manufacturing the master mold include pin-bundling techniques, laminate techniques, and direct machining techniques. Each of these techniques has its own benefits and limitations.
Several advantages can be realized by fabricating cube comer cavity based retroreflective sheeting. One advantage is the ability to use a much wider variety of material compositions for the body layer than is otherwise possible. This is because the body layer need not be optically clear--in fact it can even be opaque--unlike rear-surface body layer constructions. Another advantage is the ability to form certain types of structured surfaces in the body layer more rapidly than it takes to form a negative copy of such structured surfaces in rear-surface body layer constructions. This is because molds used to form the structured surface of a front-surface body layer can have grooves that are essentially unbounded in the direction of the groove. In contrast, molds used to form the structured surface of a rear-surface body layer typically have an array of closed (cube comer) cavities bounded by a plurality of inverted grooves, i.e., ridges. The unbounded grooves of the former molds are easier to fill with body layer material than the array of closed cavities provided on the latter molds.
Cube corner cavity based retroreflective sheeting, however, also has certain drawbacks. One of these is the grayish appearance, known as gray-cast, of the sheeting when an aluminum vapor coat is used as the reflective film on the cavity faces. Gray-cast is disadvantageous in signing applications because of its effect on perceived color of the sign, most notably the reduction of daytime whiteness. This problem can be alleviated somewhat by substituting higher reflectivity materials, such as silver, in place of aluminum. A second drawback is corrosion or other degradation of the reflective film. Unfortunately, silver is more susceptible to degradation than is aluminum. Although a cover layer can provide a certain amount of protection, harmful agents present at exposed edges of the sheeting can migrate along the reflective film, steadily advancing into the sheeting starting from such edges.
Discontinuous vapor coats applied to the structured surface of cube corner sheeting are known (see, for example, U.S. Pat. No. 5,734,501 (Smith) and U.S. Pat. No. 5,657,162 (Nilsen et al.)). Such discontinuous coatings, however, have only been disclosed in connection with rear-surface body layer-type sheeting, and are utilized to address concerns other than those of interest here.
Retroreflective sheetings that incorporate the advantages of cube corner cavity based sheetings, while eliminating or reducing the disadvantages referred to above, would have wide applicability.